TY - JOUR

T1 - Solid-state dynamics in the closo-carboranes

T2 - A B-11 MAS NMR and molecular dynamics study

AU - Ahumada, Hernan

AU - Kurkiewicz, Teresa

AU - Thrippleton, Michael J.

AU - Wimperis, Stephen

PY - 2015/3/19

Y1 - 2015/3/19

N2 - This work explores the dynamic behavior of the three closo-carborane isomers (formula C2B10H12) using modern solid-state magic angle spinning (MAS) NMR techniques and relates the experimental measurements to theoretical results obtained using molecular dynamics simulations. At high temperatures and at B-0 = 9.4 T, the B-11 MAS line widths are narrow (40-90 Hz) for the three isomers. The rotational correlation times (tau(c)) calculated by molecular dynamics are on the picosecond time scale, showing a quasi-isotropic rotation at these temperatures, typical for liquid systems. For all three isomers, the values of the B-11 spin-lattice relaxation times (T-1) show discontinuities as the temperature is decreased, confirming the phase changes reported in the literature. At low temperatures, the B-11 MAS spectra of all three isomers exhibit much broader lines. The simulations showed that the molecular reorientation was anisotropic around different symmetry axes for each isomer, and this was supported by the values of the reduced quadrupolar parameter P-Q(eff) derived from dynamic shift measurements using B-11 MQMAS NMR spectroscopy. The behavior of P-Q(eff) as a function of temperature for pi-carborane suggests that molecular reorientation is about the C5 symmetry axis of the molecule at low temperatures, and this was supported by the molecular dynamics simulations.

AB - This work explores the dynamic behavior of the three closo-carborane isomers (formula C2B10H12) using modern solid-state magic angle spinning (MAS) NMR techniques and relates the experimental measurements to theoretical results obtained using molecular dynamics simulations. At high temperatures and at B-0 = 9.4 T, the B-11 MAS line widths are narrow (40-90 Hz) for the three isomers. The rotational correlation times (tau(c)) calculated by molecular dynamics are on the picosecond time scale, showing a quasi-isotropic rotation at these temperatures, typical for liquid systems. For all three isomers, the values of the B-11 spin-lattice relaxation times (T-1) show discontinuities as the temperature is decreased, confirming the phase changes reported in the literature. At low temperatures, the B-11 MAS spectra of all three isomers exhibit much broader lines. The simulations showed that the molecular reorientation was anisotropic around different symmetry axes for each isomer, and this was supported by the values of the reduced quadrupolar parameter P-Q(eff) derived from dynamic shift measurements using B-11 MQMAS NMR spectroscopy. The behavior of P-Q(eff) as a function of temperature for pi-carborane suggests that molecular reorientation is about the C5 symmetry axis of the molecule at low temperatures, and this was supported by the molecular dynamics simulations.

KW - CRYSTALLINE ORTHO-CARBORANE

KW - PHASE-TRANSITIONS

KW - META-CARBORANE

KW - FREQUENCY-SHIFT

KW - SPECTROSCOPY

KW - RELAXATION

KW - SIMULATION

KW - SPECTRA

KW - BORANES

KW - MOTIONS

U2 - 10.1021/acs.jpcb.5b00043

DO - 10.1021/acs.jpcb.5b00043

M3 - Journal article

VL - 119

SP - 4309

EP - 4320

JO - Journal of Physical Chemistry B

JF - Journal of Physical Chemistry B

SN - 1520-6106

IS - 11

ER -